Stop inadvertently upgrading client intro connections to ntor

  o Major bugfixes (hidden services):

    - Clients require hidden services to include the TAP keys

      for their intro points in the hidden service descriptor.

      This prevents an inadvertent upgrade to ntor, which a

      malicious hidden service could use to discover which

      consensus a client has.

      Fixes bug 20012; bugfix on 0.2.4.8-alpha. Patch by teor.

  o Minor bugfixes (hidden services):

    - Stop logging intro point details to the client log on

      certain error conditions.

      Fixed as part of bug 20012; bugfix on 0.2.4.8-alpha.

      Patch by teor.

    smartlist_del(usable_nodes, i);

    goto again;

  }

  /* Do we need to look up the router or is the extend info complete? */

  /* All version 2 HS descriptors come with a TAP onion key.

   * Clients used to try to get the TAP onion key from the consensus, but this

   * meant that hidden services could discover which consensus clients have. */

  if (!extend_info_supports_tap(intro->extend_info)) {

    const node_t *node;

    extend_info_t *new_extend_info;

    if (tor_digest_is_zero(intro->extend_info->identity_digest))

      node = node_get_by_hex_id(intro->extend_info->nickname);

    else

      node = node_get_by_id(intro->extend_info->identity_digest);

    if (!node) {

      log_info(LD_REND, "Unknown router with nickname '%s'; trying another.",

               intro->extend_info->nickname);

    log_info(LD_REND, "The HS descriptor is missing a TAP onion key for the "

             "intro-point relay '%s'; trying another.",

             safe_str_client(extend_info_describe(intro->extend_info)));

    smartlist_del(usable_nodes, i);

    goto again;

  }

#ifdef ENABLE_TOR2WEB_MODE

    new_extend_info = extend_info_from_node(node, options->Tor2webMode);

#else

    new_extend_info = extend_info_from_node(node, 0);

#endif

    if (!new_extend_info) {

      const char *alternate_reason = "";

#ifdef ENABLE_TOR2WEB_MODE

      alternate_reason = ", or we cannot connect directly to it";

#endif

      log_info(LD_REND, "We don't have a descriptor for the intro-point relay "

               "'%s'%s; trying another.",

               extend_info_describe(intro->extend_info), alternate_reason);

      smartlist_del(usable_nodes, i);

      goto again;

    } else {

      extend_info_free(intro->extend_info);

      intro->extend_info = new_extend_info;

    }

    tor_assert(intro->extend_info != NULL);

  }

  /* Check if we should refuse to talk to this router. */

  if (strict &&

      routerset_contains_extendinfo(options->ExcludeNodes,